The production of glass-ceramic articles had its genesis in U.S. Pat. No. 2,920,971. As is explained in that patent, a glass-ceramic article is prepared through the controlled crystallization in situ of a precursor glass body. That preparation involves three basic steps: first, a glass-forming batch commonly containing a nucleating or crystallization-promoting agent is melted; second, the melt is simultaneously cooled to a temperature below the transformation range thereof and a glass body of a desired geometry shaped therefrom; and, third, the glass body is exposed to temperatures above the annealing point and often above the softening point of the glass to generate crystals in situ. To achieve greater uniformity in crystal size, the parent glass may frequently be initially exposed to a temperature somewhat above the transformation range to develop a myriad of nuclei in the glass, following which the temperature is raised to cause the growth of crystals on those nuclei.
Glass-ceramic products have also been prepared by firing glass frits, i.e., glasses in the form of finely-divided powders, which frequently will not include a nucleating agent in their compositions. That is, surface crystallization resulting from the high surface area presented by the very finely-divided glass powders is relied upon to promote uniformly fine-grained crystallization.
In general, glass-ceramic articles are desirably highly crystalline; U.S. Pat. No. 2,920,971 specifies at least 50% crystalline. Because of this high crystallinity, glass-ceramic articles take on physical properties more closely akin to those of the crystal phase than those of the parent glass. Moreover, the composition of any residual glassy matrix will be quite dissimilar from that of the precursor glass inasmuch as the components of the crystal phase will have been removed therefrom.
Because of the wide variety of physical properties that can be enjoyed in glass-ceramic products through the many different types of crystal phases which can be developed therein, glass-ceramics have found utility in such diverse applications as radomes, dental constructs, culinary ware, printed circuit boards, dinnerware, and matrices for storage of radioactive materials.
A combination of thermal stability, thermal shock resistance, and mechanical strength is vital when a material is to be subjected to severe thermo-mechanical environments. Dielectric requirements may also dictate that the material be essentially free from alkali metals, especially sodium.
Serial No. 380,464, filed May 20, 1982 in the names of J. J. Brennan, C. K. Chyung, and M. P. Taylor under the title GLASS-CERAMIC COMPOSITIONS OF HIGH REFRACTORINESS, describes glass-ceramic compositions in the Li.sub.2 O-MgO-Al.sub.2 O.sub.3 -SiO.sub.2 system which are capable of long term use at temperatures up to 1100.degree. C., and short term exposure to 1200.degree. C. Those glass-ceramics contained beta-spodumene and/or beta-quartz solid solution as the predominant crystal phase and had, as their principal application, service as matrices for SiC fiber reinforced composite bodies. It was observed in that disclosure that, in an oxidizing atmosphere, SiC fibers react with the matrix to deleteriously affect the strength and fracture toughness of the composite articles, primarily due to the oxidation of the SiC fibers with the concomitant generation of gaseous species resulting in fiber strength degradation. The matrix viscosity at a desired use temperature should be at least on the order of 10.sup.13 poises (the annealing point); otherwise, the load transfer through the shear strength of the matrix is too low to maintain efficient reinforcement.
As was noted above, Ser. No. 380,464 discloses glass-ceramic compositions suitable for extended use at temperatures up to 1100.degree. C. and brief exposures to temperatures up to 1200.degree. C. For certain applications, e.g., jet engine components, glass-ceramics sufficiently refractory to withstand long term exposures to temperatures up to 1300.degree. C. would be highly desirable. Also, the capability of acting as a matrix for SiC fibers, i.e., there being essentially no reaction between the matrix and the SiC fibers, would be an added plus. However, to satisfy that high temperature requirement, the glass-ceramic matrix must demonstrate such refractoriness subsequent to the crystallization in situ process that the viscosity of the body is at least 10.sup.13 poises at 1300.degree. C.
In addition, where SiC fiber-containing composites are envisioned, it is much to be preferred that the glass-ceramic exhibit a relatively low coefficient of thermal expansion and good sinterability so that the composite articles can be fabricated at relatively low temperatures and pressures (.about.1000.degree. C. and .about.1000 psi). Not only is good sinterability at relatively low temperatures desirable from the practical points of view of ease and cost of producing composites, but also higher temperatures hazard reactions taking place between the matrix and the SiC fibers.
The glass-ceramics of Ser. No. 380,464 consist essentially, expressed in terms of weight percent on the oxide basis, of:
______________________________________ Li.sub.2 O 1.5-5 Al.sub.2 O.sub.3 15-25 SiO.sub.2 60-75 ZrO.sub.2 1-5 Nb.sub.2 O.sub.5 0-10 Ta.sub.2 O.sub.5 0-10 Nb.sub.2 O.sub.5 + Ta.sub.2 O.sub.5 1-10 MgO 0-10 ______________________________________
Where those compositions are to be used to fabricate composite articles with SiC fibers, TiO.sub.2 will be essentially absent therefrom and 0.5-3% As.sub.2 O.sub.3 will be incorporated into the composition. TiO.sub.2 behaves as a flux and, hence, adversely affects the refractoriness of the product. Furthermore, TiO.sub.2 appears to form titanium silicide intermetallic compounds at the interface of the SiC fiber-matrix interface during formation of the composite body, thereby leading to reduced fracture toughness in the composite.
Arsenic, added as As.sub.2 O.sub.5 to the parent glass batch, substantially improves the resistance of the glass-ceramics to oxidation. It was hypothesized that, since arsenic can exist in two oxidation states, viz., As.sup.+3 and As.sup.+5, it acts as an oxygen buffer to trap oxygen as it migrates inwardly from the surface of the composite.
Nb.sub.2 O.sub.5 and Ta.sub.2 O.sub.5 enhance the refractory character of the glass-ceramics and were theorized to perhaps perform as secondary nucleants (ZrO.sub.2 being the primary nucleant). More importantly, however, Nb.sub.2 O.sub.5 and Ta.sub.2 O.sub.5 were discovered to provide in situ protection from SiC-glass interaction through the formation of NbC and/or TaC at the SiC-glass interface and/or the development of a very thin protective layer around the SiC fiber. Whatever mechanism is involved, the NbC and/or TaC reaction product acts to restrict active oxidation of the SiC fibers at elevated temperatures and to inhibit SiC-glass interfacial reactivity. As a result, the Nb.sub.2 O.sub.5 and/or Ta.sub.2 O.sub.5 content in the glass-ceramic matrix will be reduced to the extent of the carbide layer.
To secure highly crystalline bodies wherein the crystals are quite uniformly fine-grained, the compositions will contain 2-3.5% Li.sub.2 O, 1.5-6% MgO, and 1-3% ZrO.sub.2.